What's the impedance of these speakers?
Also, the voltage sensing, yes I think you should be able to make a circuit to handle that. What the resistors do is they feed a certain current or voltage into the Tripath chip, if I understand correctly. An active circuit is certainly capable of doing that. The question is however, what would the active circuit do? I'd just try to set a usefully wide range of operational voltage and leave it be.
All the speakers are nominally 8 ohms, which is what the Amp 4 wants.
Unfortunately there's a formula for the feedback resistor based on voltage; there's no adjustment for "q" or range so I'm stuck with setting it to the maximum expected voltage. The question is how much it matters; I don't mind a bit of distortion as this is not ultra hifi territory.
Ahh, yes, the feedback resistor.
I don't think there will be that much of a distortion problem. It's more a question of optimality than correctness. You should be able to work with the level fed into the amp, possibly trading in some noise for distortion. (If I understand correctly.)
The way I understand the modulator gain resistor is that you want the input stage to be swinging at around 4VPP, and the output stage at an appropriate place near the rail voltage. This is AFAIK because the nicest sound is at around 4VPP at the input. (Not sure? Might be something else.)
Now if you set the input feedback to give the proper 4VPP, then the modulator gain resistors can cause the amp to not use all the available rail voltage, or it can smash the signal into the rails if the modulator gain is set too high for the rail voltage at that moment. But then you can always decrease the signal strength at the inputs. Then the input stage is swinging a somewhat less perfect voltage, under 4VPP, but you're getting all the power of the amp and not at all much distortion from the input stage. The difference is probably in under a percent of THD.
Note! -- This is a lot of speculation on my part, but I believe I have the principles down OK.
To use all available rail voltage, set modulator gain resistors to match highest operational voltage.
To cure output stage distortion when rail voltage is lower than that, lower input signal.
Well I've built my Amp4 now and clamped it into the bottom of its aluminium box so I can test it properly. I was worried that the earth loop caused by the piano being powered from the same source (via the 12V switching regulator) might cause problems, but this doesn't seem to be the case. (I've only tried it indoors, pedalling myself, where the noise of the generator on a suspended floor would drown out any minor interference in the speakers, though.)
I improved on the relay driver circuit to make it work over a wide range of voltage and more efficiently. I found that at the lowest voltage before undervoltage muting (about 14.9V), the maximum value of resistor I could put in series with the relay coil to hold the contacts closed was 2.7k, so for a safety margin I used 1.5k. I then found that a 470uF capacitor in parallel with this resistor allowed the coil to pull the contacts across at the lowest voltage where muting is released (about 16V), so used two 470uF capacitors in parallel for a safety margin.
I'll use another relay to disconnect everything from the generator at 32V or so for overvoltage protection. This relay can also have a secondary function: because the box is big enough to take the guts of an old Dell power brick (which I will tweak to 25V output if I can), when plugged into the mains it can switch the power over to the power brick.
I'll also replace the LM317 9V pre-regulator with the 12V output from the switching regulator, to improve efficiency a bit more.
As for speakers, I settled on the StudioSpares Forte (http://www.studiospares.com/Installa...ir/invt/248620), of which a colleague had one to let me try, but which are still out of stock. So I was about to get the Fortissimo 10P (http://www.studiospares.com/PA-Speak...le/invt/248200) instead (a proper PA speaker and 2dB more sensitive - not much - but bigger, heavier and more expensive) when I looked at the weather forecast for the weekend and decided we probably wouldn't be singing in the park after all!
You'll be making quite the racket!
Thanks for the info, the power stuff you're discussing is already useful to me
You're welcome - and thank you in turn for your advice.
If anyone's interested, there's now a page about the bicycle generator/Amp 4 combo at http://collierswoodchorus.org.uk/pedalpower.html
Great PedalPower! Would love if you write us a Featured Project article!
How does it sound?
Nice and loud and clear - as well as allowing the audience to hear the piano, it means that the choir (about half of them present in that photo) or someone making announcements really projects and attracts attention. It also means they behave themselves because their idle chit-chat is broadcast! The speakers being "installation" rather than "PA" are more geared for a nice sound (as much as you can get from plastic speakers anyway!) rather than raw power - ideal for small events rather than rock-concert levels.
Originally Posted by krilli